x86,irq: Clean up __clear_irq_vector

x86,irq: Clean up __clear_irq_vector

Fix and clean up the logic to __clear_irq_vector().

We always need to clear the things related to cfg->vector.

If the IRQ is currently in motion, then we need to also clear
out things related to cfg->old_vector.

This patch reorganizes the function to make the parallels between
the two clean-ups more obvious.

The main functional change here is with cfg->used_vectors; make
sure to clear cfg->vector always (even if !cfg->move_in_progress);
if cfg->move_in_progress, clear cfg->old_vector as well.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>